Class 6 Lecture Demo: Map Projections

Why Map Projections Matter:

Map Projections Impact Area, Shape, Distance & Direction

  • Issue #1:

    • A map projection is a mathematical formula used to transfer all or part of the curved surface of the earth onto the flat surface of a map.

  • Issue #2:

    • Conversion of the spherical coordinates to a Cartesian coordinate system is necessary if we want to measure distances in meaningful units such as meters, feet or miles or if we want to estimate the area of a land unit.

Levels of Geographic Abstraction for Map Projections:

  • Process:

Earth to Map

  • Earth Surface:

Earth Model

  • Geoid:

Position of Geoid

Geoid

  • Ellipsoid:

Ellipsoid

Ellipsoid

Multiple Ellipsoids, Different Mapping Purposes

  • Development Surface + tangential point:

Point of Tangency

3 Different Development Surfaces

  • Datum:

  • A datum is a formal description of the shape of the Earth along with an “anchor” point for the coordinate system.

  • Geographic coordinate systems consist of a datum, a reference ellipsoid, and an angular unit of measure.

Datum Notation on a Reference Map

Datum Selector in a Typical GPS Unit

Topic 1: How to choose a projection:

  • What distortion can you give up in favor of the one(s) you really need?

  • How big is your project area?

  • What unit of measure (feet, inches, degrees…) would you like your map to be measured in?

  • What is the end goal of your project?

Topic 2: CRS Parameters in QGIS

  • CRS Overview:

QGIS CRS Properties

  • CRS Expressions:

CRS Formats

  • proj4:

proj4

Topic 3: EPSG Lookup

Find a CRS in QGIS

  • Question Icon = No CRS:

A Question Mark Icon means QGIS is unable to determine the Layer CRS

Topic 4: Project vs Layer CRS:

Projects have an underlining CRS; layers can have the same CRS, or different CRS within One Project

Project Properties CRS

  • QGIS supports “on the fly” CRS transformation for both raster and vector data.

  • Selecting a CRS, both project and layer:

CRS Selector

Demo Lab 1: Tissot’s indicatrix

Tissot’s indicatrix

  • Default WGS84:

Preview CRS within QGIS

  • Transformation to World_Polyconic:

EPSG 54021

Demo Lab 2: CRS + Missing CRS scenario:

Layer Properties > Missing CRS

Warning in QGIS Layers Panel for Missing CRS

Setting a CRS in QGIS

Searching for WGS84 in the QGIS CRS Selector Interface

Demo Lab 3: Reprojection

Reprojecting a Feature within QGIS

Searching for WGS84 in the QGIS CRS Selector Interface

  • In order to correctly project data into a specific target CRS, either your data must contain information about its coordinate reference system or you will need to manually assign the correct CRS to the layer.

Demo Lab 3: CRS + Areal Measurements

  • As default, ESPG 4326 WGS 84 project CRS will be used to calculate areal units via Field Calculator. The Field Calculator will use $area to determine areal calculations in square meters. In the current QGIS LTR, the default areal unit for calculations is indeed square meters unless it is changed to a different unit:

Default Measurement Units in Current QGIS LTR

Geometry Attributes

  • IF Layer CRS is chosen when creating Geometry Attributes, the resulting area calculations will be in degrees based on ESPG 4326 WGS 84 layer/project crs:

Forcing Layer CRS

Resulting Area expressed in Degrees

  • Using project and/or layer crs for the Vermont State Plane - EPSG 102745, Areal calculations can be determined in square feet as the projection unit is feet:

Setting CRS to Vermont’s State Plane Coordinate System

Geometry Attributes based on Layer or Project CRS will result in Area based on Square Feet

  • Square Meters and Square Feet can be expressed as Square Miles using the following formulas, respectively:

    • mi² * 0.00000038610

    • ft² * 0.000000035870

Demo Lab 4: Datum Transformations

Datum Transformations

Datum Transformations